Just a note, but the original question was how does one become a mod. It wasn't one like, "I want to become a mod, how do I do it?"
In my mind, that's two different questions. Though both might show that an individual is interested in being a Mod, the second one shows an overzealousness that has gotten forums in trouble in the past.
There's been lots of good answers in this thread, too.
Though we usually want a minimum post count and a minimum time as a member, remember that post counts don't add up in some forums. The minimums there show that you've been around here for awhile and you should know the way the forum is run and the rules.
Another thing that some people seem to forget when it comes to Mods is that if you have been a "problem" in the past, especially a continual problem, then you'll probably never make it.
So for those that are interested, just be helpful, know what you're talking about, and remember that being a Mod adds alot of responsibility to your time at the forum.